I've got a very complex database with a mixture of SQL Generated Data and manually entered/real data. I've added a couple of columns to one of the tables - just Firstname Lastname and I want to use SQL Data Generator to populate only these two columns.

The workaround to this kind of problem is to restore a temporary copy of your database and use this as the source data for all the columns you want to keep the current data in. You can then use the generators on the remaining new columns. It's obviously not an ideal solution but the end result is the same.
There's an article that covers these ideas here